Heat Transfer Patterns Research Articles

LAMINAR NATURAL CONVECTIVE HEAT TRANSFER IN PARABOLIC ENCLOSURES

A finite volume analysis of laminar natural convection of air within full parabolic enclosures heated from below is reported for the enclosure aspect ratio 0.27 £ AR £ 0.7 and a range of the Rayleigh number 5.5 x 106 £ Ra £ 9.8 x 107. The shape of the enclosures strongly affects the results obtained. Quasi-symmetrically arranged counter-rotating vortices are formed in each of the enclosures. The shape of the cells are influenced by the bounding curved and flat surfaces as they change from being rectangular at the periphery to circular at the core. There are reduction in the size and intensity of the on the cells from the center to the bottom corners. At steady state, there exists in each enclosure a central thermal plume that thickens as the roof pitch angle increases. The heat transfer pattern matches the flow of the plumes and jets from the walls. For the 15o roof pitch enclosure, the heat transfer rate increases sharply at a critical Ra value of 5 x 106.

Wall temperature effects on shock unsteadiness in a reattaching compressible turbulent shear layer

In this study, the effect of heat transfer on the compressible turbulent shear layer and shockwave interaction in a scramjet has been investigated. To this end, highly resolved Large Eddy Simulations (LES) are performed to explore the effect of wall thermal conditions on the behavior of a reattaching free shear layer interacting with an oblique shock in compressible turbulent flows. Various wall-to-recovery temperature ratios are considered, and results are compared to the adiabatic wall. It is found that the wall temperature affects the reattachment location and the shock behavior in the interaction region. Furthermore, fluctuating heat flux exhibits a strong intermittent behavior with severe heat transfer compared to the mean, characterized by scattered spots. The distribution of the Stanton number shows a strong heat transfer and complex pattern within the interaction, with the maximum thermal (heat transfer rates) and dynamic loads (root-mean-square wall pressure) found for the case of the cold wall. The analysis of LES data reveals that the thermal boundary condition can significantly impact the wall pressure fluctuations level. The primary mechanism for changes in the flow unsteadiness due to the wall thermal condition is linked to the reattaching shear layer, which agrees with the compressible turbulent boundary layer theory.

Heat transfer and flow pattern map development of R134A in a U-bend tube for flow boiling evaporation

This study presents flow boiling evaporation heat transfer and flow patterns development of R134a in a U-bend tube with the curvature ratio of 2.55 for the downward flow orientation. The volume of fluid (VOF) multiphase model in the computational fluid dynamics (CFD) code Ansys Fluent® was utilized and incorporated with phase change model for volume fraction tracking of each of the phases. The simulations were conducted under these operating conditions: (mass fluxes from 200 to 600 kg/m2s, vapour qualities from 0.01–0.9 and 10 kW/m2 of heat flux). The relationships of mass flux with respect to vapour quality on heat transfer and frictional pressure drop in the U-bend tube were analysed. The effect of mass flux with respect to vapour quality on heat transfer coefficient showed that heat transfer coefficient increases as mass flux increases and with increase in vapour qualities at constant mass flux which later decreases at the higher vapour qualities due to dry-out. The frictional pressure drop was found to increase as mass flux and vapour quality increases as a result of increase in an interfacial shear stress of liquid and vapour phases. At higher vapour qualities, frictional pressure drops decrease due to drastic reduction of the liquid film. The influence of the bend was determined by the development of flow pattern map before and after the bend. The numerical computation results agrees with what is obtained in the open literature.

The influence of anionic surfactant in water on the quenching process

Understanding the patterns of heat transfer during quenching is important for many technical applications. Of particular interest is the boiling regime, which is characterized by high intensity and occurs at surface temperatures exceeding the temperature of attainable liquid superheat. This work is aimed at studying the effect of surfactants on the onset of intense heat transfer during quenching. For this, experiments were carried out on quenching spheres made of different metals (nickel, stainless steel and zirconium) in water with different concentrations of surfactants. The surfactant was alkylbenzene sulfonate, the concentration of which varied from 0.1 to 2%. The analysis of the obtained cooling thermograms revealed the influence of not only the surfactant concentration on the beginning of the intensive cooling mode, but also the state of the heat transfer surface.

Development of Nusselt correlation for heat transfer studies in vertical transport duct using limestone and chalcopyrite for waste heat recovery

AbstractProcess industries deal with high energy inputs and often generate high temperature exit gases, which can potentially be considered as secondary energy sources. If such waste heat is recovered, considerable savings in the net energy demand is possible. This report, based on this objective, details the outcome of an experimental study conducted in a vertical transport duct (riser) where solid particles, through a co‐current contact with hot gas, recover the energy content of the gas. Data relevant to heat transfer patterns are collected as not much information is available in the literature on these aspects. Empirical correlation for heat transfer coefficient (Nusselt Number) is proposed as . Fabricated experimental setup is a prototype of suspension preheater (riser – cyclone combination) used in a modern day cement industry but the correlation can be extended to diverse other industries, where waste heat recovery is possible.

Poiseuille-Rayleigh-Bénard mixed convection flow in a channel: Heat transfer and fluid flow patterns

In the present work, a numerical and experimental study of mixed convection of water flow in a horizontal channel subjected to a constant heat flux is presented. An experimental setup was constructed to delineate the fluid flow patterns inside the channel, while a numerical simulation was performed to study the heat transfer characteristics of the flow. This study is performed for Rayleigh and Reynolds numbers in the range of 104 ≤ Ra≤ 106 and 25≤ Re≤ 100, respectively. The present predicted results are in good agreement with the experimental ones. The longitudinal evolution rolls revealed the existence of four zones corresponding to the entry zone, establishment zone, destabilization zone, and turbulent zone. Moreover, numerical simulations showed that the Nusselt number for the mixed convection gives higher heat transfer coefficient compared to forced convection. In addition, new correlations for the Nusselt number have been developed for the first time for the establishment length, onset and established zones as a function of Rayleigh and Reynolds numbers.

Analysis of phase change heat transfer with annulus 1 kW class methanol steam reforming reaction under high-pressure operation

A fuel cell air independent propulsion (AIP) system of underwater vehicle requires a hydrogen storage system. The methanol steam reforming system is a candidate of hydrogen storage which can produce hydrogen from chemical reaction. Different from reforming system for station fuel cell system, the methanol steam reformer (MSR) for underwater vehicle requires high-pressure operation.Since the longitudinal temperature uniformity is a core parameter of conversion efficiency of steam reforming system, this study is focused on computational analysis of phase change heat transfer through the annulus for methanol steam reforming reaction. The annulus MSR using phase change material was developed to improve the temperature uniformity. The simulation model is verified with safety and performance analysis code (SPACE). The performance parameters of MSR were flow arrangement, steam to carbon ratio (SCR), and gas hourly space velocity (GHSV). The results were analyzed in terms of the hydrogen yield, heat flux, liquid mass flow rate, and methanol conversion rate. The flow arrangement varied the methanol conversion rate to a minor extent of approximately 0.1% because wall temperature was maintained uniformly. In the case of SCR, the hydrogen yield at SCR 2.5 was 0.637 (dry basis), which was the highest yield rate. Also, if GHSV was increased, hydrogen yield decreased from 0.690 (dry basis) to 0.527 (dry basis). The heat transfer pattern was also analyzed and it was found that steam is interactively condensed along with the progress of the reforming reaction.

Hybrid Simulation of Turbulent Natural Convection in an Enclosure with Thermally-Conductive Walls

This paper analyzes the interaction of high Rayleigh number flow with conjugate heat transfer. The two-relaxation time lattice Boltzmann is used as a turbulent buoyancy-driven flow solver whereas the implicit finite difference technique is applied as a heat transfer solver. An in-house numerical code is developed and successfully validated on typical CFD problems. The impact of the Biot number, heat diffusivity ratio and the Rayleigh number on turbulent fluid flow and heat transfer patterns is studied. It is revealed that the thermally-conductive walls of finite thickness reduce the heat transfer rate. The temperature of the cooled wall slightly depends on the value of the buoyancy force. The heat diffusivity ratio has a significant effect on thermal and flow behavior. The Biot number significantly affects the mean Nusselt number at the right solid–fluid interface whereas the mean Nusselt number at the left interface is almost insensible to the Biot number variation.

Field test and numerical investigation on thermal environment of tunnel with air layer structure

In this study, an air layer structure is proposed for controlling the thermal environment in high ground temperature tunnels, and for meeting the environmental requirements during tunnel construction. Thermal response field experiments on a mountain tunnel equipped with an air layer structure were conducted under tunnel ventilation and heating. The variations in air temperature inside the tunnel and thermal response characteristics of the surrounding rock at the condition of tunnel with an air layer structure were analyzed. Moreover, the effects of the tunnel ventilation and type of air layer on the thermal insulation performance of the air layer structure were evaluated in detail. Based on the theoretical analysis, the heat transfer pattern of the air layer structure was discriminated. Good consistency was found between the theoretical and experimental results for the discrimination of the thermal insulation performances of different types of air layers. Finally, the heat transfer process of the tunnel with the air layer structure was simulated and analyzed by establishing a numerical model. The results show that the numerical results regarding the temperature fields within the tunnel are in good agreement with the measured results, and that a high ground temperature tunnel with an air layer structure has a preferable thermal insulation effect.

Onset of Boiling, Heat Transfer, and Flow Patterns of Flow Boiling on the Superhydrophobic Porous Copper Surface in a Microchannel

Abstract To explore the effect of microstructured porous surface on enhancing the heat transfer of flow boiling in a microchannel, the microporous copper surface was fabricated with microscale pores in ranges of 1–5 μm, which presented super-hydrophobicity. Subcooled flow boiling experiments were carried out to study the hydraulic and thermal transport performance in a single narrow rectangular microchannel. High-speed flow visualizations were conducted coupled with instrumental measurements to illustrate the effects of heat flux and mass flux on heat transfer performance and flow patterns for originally hydrophilic bare copper surface and superhydrophobic microporous structured surface. The onset of boiling (ONB) characteristics of both test surfaces was compared with predictive correlations with a good agreement and verified by the captured flow pattern images. Benefit from the superhydrophobic wettability provided by its microscale porous structures and a large number of potential nucleation sites, the required wall superheats, and imposed heat fluxes of onset of boiling both decreased for the modified surface. The flow patterns on the two surfaces varied, which resulted in the different trends of heat transfer coefficient (HTC) with mass fluxes and heat fluxes. Because of the strengthened bubble departure process, the enhancement of the porous surface compared to the original bare surface gradually increased with mass fluxes. The average two-phase heat transfer coefficients of the superhydrophobic porous copper surface were enhanced for up to 74.84%, due to the earlier onset of boiling, higher frequency of bubble nucleation, and strengthened boiling intensity.

Aerothermal Optimization of Turbine Squealer Tip Geometries With Arbitrary Cooling Injection

Abstract The optimization of the turbine rotor tip geometry remains a vital opportunity to create more efficient and durable engines. Balancing the aerodynamic and thermal aspects, while maintaining the mechanical integrity, is key to reshape one of the most vulnerable and life-determining parts of the entire turbine. The ever-increasing turbine gas temperatures, combined with the difficulty of cooling the tip and the aerodynamically penalizing nature of the overtip leakage vortex, make the design of the tip a truly multidisciplinary challenge. While many earlier efforts focused on uncooled geometries or studied the aerothermal impact with a fixed cooling configuration, the current paper presents the outcome of a multi-objective optimization where both the squealer rim geometry and the cooling injection pattern were allowed to vary simultaneously. This study explores a significantly wider design space, seeking a further synergistic aerothermal benefit through the combination of a quasi-fully arbitrary cooling arrangement, with mutating squealer rim structures. Specifically, the current manuscript presents the results of over 330 cooled and uncooled squealer tip geometries. The high-pressure turbine tip was automatically altered using a novel parametrization strategy adopting a maximum of 40 design variables to vary the squealer rim structures, as well as the size and location of the various cooling holes. The aerodynamic and thermal characteristics of every design were evaluated through Reynolds-averaged Navier–Stokes computational fluid dynamics (CFD) simulations with the k–ω shear-stress transport (SST) model for the turbulence closure, adopting an unstructured hexahedral grid typically containing more than 8 × 106 cells. A multi-objective differential evolution algorithm was used to obtain a Pareto front of designs which maximize the aerodynamic efficiency, while minimizing the overtip thermal loads. Eventually, a detailed investigation and robustness study was performed on a set of prime squealer geometries, to further investigate the aerodynamic flow topology and the effect of various cooling injection schemes on the heat transfer patterns.

Heat transfer measurements of a turbine endwall with engine-representative freestream turbulence and inlet swirl

ABSTRACT Flow distortions from upstream combustors generate various and significant changes to the patterns of endwall heat transfer for an aircraft engine. In this study, to model engine-representative oncoming flows into the high-pressure turbine, turbulence and swirl generators are carefully designed at the inlet of a linear turbine cascade. Detailed heat transfer characteristics over the endwall are measured by using a steady thermochromic liquid crystal (TLC) method for various Reynolds numbers. Comparisons with a baseline case that has no upstream turbulence or swirl generators show that both turbulence and inlet swirl alter the heat transfer levels and distribution patterns, but their mechanism of action differs from one another. The turbulence alters the heat transfer by thinning the inlet boundary layer, while the inlet swirl generates the changed heat transfer due to the sweep effects of the swirl on the endwall, resulting in a much higher heat transfer enhancement than that by the turbulence. Additionally, the highest enhancement for both cases is found at the low Reynolds number of 1.0 × 105 and higher Reynolds numbers yield to a reduced enhancement. Specially, compared to the turbulence case, the enhancement by the inlet swirl drops much more rapidly, indicating that the inlet swirl effects have stronger links to the Reynolds number.

Convective heat transfer mechanisms of molten phase change material in a vertical slender rectangular cavity: A numerical case study

In this paper, the convective heat transfer pattern of molten phase change material (mPCM) in a slender rectangular cavity with constant heat flux boundary condition at one side and flow boundary condition at opposite side is numerically investigated. The influences of the Rayleigh number (constant heat flux boundary condition, Ra = 104, 105 and 106), the Reynolds number (flow boundary condition, Re = 1×104, 2×104, 3×104, 4×104 and 5×104) and aspect ratio (AR = 5, 7.5 and 10) on the convective heat transfer mechanisms of the mPCM are analyzed. Several dimensionless parameters are used as quantitative indexes to describe and evaluate the convection and heat transfer process in the cavity. The results show that the convective heat transfer efficiency can be improved by increasing the Ra or Re. With longer length of the cavity in the direction of buoyancy, the internal natural convection heat transfer of mPCM is more intense. Especially, due to different boundary conditions, the heat transfer intensity at two vertical walls of the cavity is different. In addition, the mixing degree of cold and hot fluid in the cavity will also affect the local heat transfer effect at the wall.

Bio-convective Darcy-Forchheimer periodically accelerated flow of non-Newtonian nanofluid with Cattaneo–Christov and Prandtl effective approach

The thermal applications of nanofluids significantly improved the heat and mass transfer pattern which convey necessary role in many engineering and industrial zones. The consideration of nanofluids contributes many dynamic applications in the solar energy and thermal engineering problems. Moreover, the stability of nanofluids is enhanced perfectively with motile microorganisms which have applications in petroleum sciences, biofuels, bio-engineering, bio-medical, enzymes etc. This research determines the applications of bio-convection in Casson nanoliquid flow subject to the variable thermal conductivity and inertial forces. The Cattaneo–Christov relations are treated to modify the heat and concentration equations. The accelerated surface with sinusoidal type velocity induced the flow. The flow problem is formulated in terms of partial differential equations. The homotopic scheme is followed in order to suggest the analytical relations. After highlighting the convergence region, the graphical simulations with help of MATHEMAITCA are performed. The physical output is addressed in view of all flow parameters. The 3-D behavior of velocity, temperature, concentration and microorganisms is also addressed.

Comparison of 1-D analytical solutions and a numerical model for quantifying hyporheic exchange flux using the temperature tracer method

Recently, the use of temperature tracer methods to investigate hyporheic exchange in rivers has attracted widespread attention. The quantification of the hyporheic exchange flux using temperature has been a topic of interest. Based on a summary and elaboration of the one-dimensional (1-D) analytical models for quantifying the hyporheic exchange flux based on the temperature tracer method, a hydrothermal coupling numerical model for the hyporheic zone considering the overall model of a river is further constructed. Combined with the water level and temperature time-series monitoring data collected from relevant rivers in the Walker Lake Basin in the United States, the effectiveness of the constructed numerical model in simulating the temperature and seepage patterns in the hyporheic zone is verified. Based on the verified hydrothermal coupling numerical model, the differences in the quantification of the vertical hyporheic exchange flux between different models are compared and analyzed. In addition, a sensitivity analysis based on the orthogonal test method is carried out to investigate the uncertainty of the 1-D analytical solution model. Finally, the advantages and disadvantages of the 1-D analytical model and the numerical model are discussed from the perspective of theoretical analysis. The results show that the constructed numerical model can be used to effectively characterize the heat transfer and fluid flow patterns in the hyporheic zone. When the result from the numerical model is not considered, the l-D analytical model based on the Hatch amplitude method is an ideal model for quantifying the vertical hyporheic exchange flux using temperature. The sensitivity analysis results reveal that the volumetric heat capacity of the soil (Cs) and the porosity (n) are relatively sensitive parameters for the Hatch solution.

Impact of Thermal Properties on Crystalline Structure, Polymorphism and Morphology of Polymer Matrices in Composites.

Morphological analysis at different levels is fundamental to understand properties of materials, as these latter are dictated not only by the chemical composition but also by the shape. Solid structures arise from a balance between thermodynamic and kinetic factors, which, especially for polymer composites, depend also on interactions amongst components. In particular, morphology is strongly affected by the heat transfer pattern during crystallization and by the difference in thermal behavior between polymer matrix and filler. Polymers show a spherulitic structure, arising from the start of crystallization in several points of the liquid phase. Within a general rounded shape, spherulites show variability in growth patterns, morphology, and geometry of boundaries. The appearance and the number of spherulites, as well as their growth mechanism, may vary not only in dependence of the chemical composition and the crystalline structures but also, for a same polymer, in consequence of experimental conditions and incorporation of fillers. This article reviews the crystallization process of polymer matrices in the framework of crystal growth and heat transport theories, and explains microstructural differences between composites and neat matrices on the basis of the differences in thermal capacity and conductivity between polymers and additives.

Two-phase R1234yf flow inside horizontal smooth circular tubes: Heat transfer, pressure drop, and flow pattern

A literature review on the two-phase flow inside plain tubes, especially for the refrigerant R1234yf, is conducted in this research. The flow boiling, condensation, and visualization of this low GWP refrigerant were examined experimentally to measure the heat transfer coefficients and pressure drop of the refrigerant, and also to present a flow pattern map. The measurements were carried out on smooth circular tubes with internal diameters of 8.2 mm for the mass velocity ranging from 95 to 410 kg m−2 s−1, vapor qualities from 0.12 to 0.92, and saturation temperatures from 26.3°C to 33.5°C. For the range of test parameters, the flow patterns were observed and compared with the work of Wojtan et al. Also, the experimental results were analyzed to evaluate the effect of mass flow and vapor quality. Generally, both parameters increase the heat transfer and pressure drop in the boiling and condensing flows. Also, the experimental results were compared with the existing models proposed to predict the heat transfer coefficient and pressure drop. Two correlations, based on the previous studies, were proposed to improve the accuracy of the heat transfer coefficient prediction. the correlation of Dobson et al. was adapted and resulted in a mean absolute error of 11.9%. Also, the Liu and Winterton model for boiling was modified to reduce its mean absolute error from 25.6% to 13.4%.

Heat transfer, pressure drop and flow patterns of flow boiling on heterogeneous wetting surface in a vertical narrow microchannel

The micro-structured surfaces have a significant impact on the flow patterns and heat transfer mechanisms during the flow boiling process. As illustrated by previous studies, there is a trade-off between a hydrophobic and a hydrophilic surface. To examine the effect of heterogeneous wetting surfaces on the flow boiling process, an experimental investigation of flow boiling in a rectangular vertical narrow microchannel with the heterogeneous wetting surfaces and original silicon surface was conducted with deionized water as the working fluid. Hydrophobic patterns perpendicular (HC) and parallel (HP) to the flow direction are compared. The subcooled boiling curves, heat transfer coefficient, and pressure drop were discussed with the variation of heat flux and mass flux, the trends of which were analyzed along with the flow patterns. The heat transfer coefficient of flow boiling on heterogeneous wetting surface HC was enhanced for up to 39.55% compared to a silicon surface with less pressure drop. During the boiling process, the dominant heat transfer mechanism was nucleate boiling, with numerous nucleate sites between the hydrophilic/hydrophobic stripes. The easily formed bubble can be easily detached due to the supply of liquid from the hydrophilic regions nearby. With the increasing wall heat flux, the boiling heat transfer coefficient of the heterogeneous wetting surface HP is gradually lower than HC due to hydrophobic stripes parallel to the flow direction cannot restrict the bubbles from expanding in the axial direction.

Nonsteady Heat Loses to the Ground from a Building with Heated Underground Floor or Slab on Ground

Numerical model and research results of nonsteady heat transfer between building and ground basement are in the article. Two most common cases of heat contact between building and ground basement are compared: building with heated underground floor and building on the slab on ground. Nonsteady heat loses over period about several years of building exploitation are analyzed that made possible to establish a patterns of heat transfer between building and ground with taking into account of ground around of building inertness. Heat loses from building represented using mean values at the one unit of length of building perimeter. In case of building with underground floor calculated nonsteady heat loses of floor and underground wall are shown. Also shown results of nonsteady heat transfer to ground from outdoor air and groundwater. Numerical methodic used in research based on explicit finite-difference method applicated to boundary value problem of nonsteady heat transfer in ground. The problem taking into account existence of moveable groundwater without heat transfer calculation in the volume of groundwater.

CFD modeling and validation of heat transfer inside an autoclave based on a mesh independency study

Autoclave processing is the main technology used in the manufacturing of structural aerospace composite parts. To optimize the autoclave process, the thermal behavior of the part and mold can be investigated through simulations. Computational fluid dynamics (CFD) provide a significant contribution to studies on heat transfer and airflow patterns, which are key points in an optimization applied to achieve a homogeneous temperature distribution inside composite parts. The solution is produced by solving the 3 D unsteady Navier–Stokes equations. This paper describes a systematic numerical study using the CFD approach to significantly improve the modeling efficiency of the heat transfer coefficient (HTC) inside an autoclave and maintain a high level of accuracy. Considering the modeling cost, calculation time, and accuracy of the results, a reasonable hybrid mesh is used based on a mesh independency study. The level of grid independence is examined using the general Richardson extrapolation method. In addition, a more robust autoclave model is presented, which is unaffected by the inlet turbulence. Further, the inlet fluid velocity and turbulence models have been identified as sensitive influencing factors. In this study, the Spalart–Allmaras turbulence model shows the best performance compared with the standard [Formula: see text] and [Formula: see text] SST models. Finally, the results are validated with the experimental data. The mean error of the simulated temperatures in the calorimeter for the front, middle and rear positions are [Formula: see text]C, [Formula: see text]C, and [Formula: see text]C, indicating a good agreement with the experiments. This paper provides guidelines on the use of a CFD simulation to predict the heat transfer during the autoclave curing process with high accuracy and reduced numerical effort.